1. Field of the Invention
The present invention relates to a composite processing apparatus and method, and more particularly to a composite processing apparatus and method useful for flattening a surface of an electric conductor (conductive material), such as copper, embedded in fine interconnect recesses provided in a surface of a substrate, in particular a semiconductor wafer, thereby forming embedded interconnects.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistivity and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in a surface of a substrate. Various techniques are known for forming such copper interconnects, including chemical vapor deposition (CVD), sputtering, and plating. According to any such technique, a copper film is formed in the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 1A through 1C illustrate a sequence of steps in an example process for forming such a substrate W having copper interconnects. As shown in FIG. 1A, an insulating film 2, such as an oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer 1a on a semiconductor base 1 on which semiconductor devices are formed. Contact holes 3 and trenches 4 are formed in the insulating film 2 by performing a lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the insulating film 2, and a seed layer 7, as an electric supply layer for electroplating, is formed on the barrier layer 5 by sputtering, CVD, or the like.
Then, as shown in FIG. 1B, copper plating is performed on the surface of the substrate W to fill the contact holes 3 and the trenches 4 with copper; and, at the same time, a copper film 6 is deposited on the insulating film 2. Thereafter, the copper film 6, the seed layer 7 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact holes 3 and the trenches 4, and the surface of the insulating film 2 lie substantially on the same plane. Interconnects composed of the copper film 6 are thus formed in the insulating film 2, as shown in FIG. 1C.
Components in various types of equipment have recently become finer and have required higher accuracy. As sub-micron manufacturing technology is becoming common, the properties of materials are more and more influenced by the processing method. Under these circumstances, in a conventional machining method in which a desired portion in a workpiece is physically destroyed and removed from a surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing and electrolytic polishing, have been developed in order to solve this problem. In contrast, with the conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects, such as formation of a damaged layer and dislocation, due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
An electrolytic processing method using an ion exchanger has been developed. This method comprises bringing an ion exchanger mounted on a processing electrode and an ion exchanger mounted on a feeding electrode into contact with or close to a workpiece, and applying a voltage from a power source to between the processing electrode and the feeding electrode while supplying a liquid, such as ultrapure water, between the processing and feeding electrodes and the workpiece from a liquid supply section to carry out removal processing of a surface layer of the workpiece.
Such a conventional electrolytic processing using an ion exchanger involves the following problems. A processed material is taken in the ion exchanger during electrolytic processing, and there is a limit on the take-in amount of processed material per unit time. Further, there is a need for regeneration or a change of the ion exchanger, which will lower the throughput. In the case of electrolytic processing (polishing) of a copper film using an ion exchanger and electrodes (a processing electrode and a feeding electrode), copper is considered to be directly taken in the ion exchanger. In some cases, however, a passive film of e.g. Cu2O or CuO is formed in the surface of a copper film during electrolytic processing. Such a passive film is physically soft and is non-conductive and, therefore, poorly removed by electrolytic processing. The conventional electrolytic processing also entails the problem of the formation of pits (small holes) in a processed surface of a workpiece depending upon the type of the workpiece, the processing conditions, and the like.
A chemical mechanical polishing (CMP) process, for example, generally necessitates a complicated operation and control, and needs a considerably long processing time. In addition, a sufficient post-cleaning of a polished surface must be conducted after the polishing treatment. This also imposes a considerable load on the slurry or cleaning liquid waste. Accordingly, there is a strong demand for omitting CMP entirely or reducing the load upon CMP. Also in this connection, it is to be pointed out that though a low-k material which has a low dielectric constant is expected to be predominantly used in the future as a material for the insulating film, the low-k material has a low mechanical strength and, therefore, has difficulty enduring the stress applied during CMP processing. Thus, also from this standpoint, there is a demand for a process that enables the flattening of a substrate without giving any stress thereto.
Further, a method has been reported which performs CMP processing simultaneously with plating, viz. chemical mechanical electrolytic polishing. According to this method, the mechanical processing is carried out to the growing surface of a plating film, causing the problem of denaturing of the resulting film.
In the case of the above-mentioned conventional electrolytic processing or electrolytic polishing, the process proceeds through an electrochemical interaction between a workpiece and an electrolytic solution (aqueous solution of NaCl, NaNO3, HF, HCl, HNO3, NaOH, etc.). Though a glossy surface or mirror surface can be formed with these methods, a uniform or even surface of sub-micron level cannot be obtained. This holds true for composite electrolytic polishing for electrolytically polishing using a slurry of an electrolytic solution containing abrasive grains.